Results
Transient Expression of PCR-fragments in Epidermal Cells
The components designed in the cassette of a PCR-fragment include a promoter, such as CaMV 35S promoter [16] or UBQ10 promoter [17], coding sequences of the gene of interest (CDS), and the terminator (NOS) (Figure 1A). Initially, PCR-fragments were generated through PCR amplification from the template plasmid DNA which contains the backbone of vector pUC18. The pair of universal primers PVU-F (located at 180 bp upstream of the promoter) and PVU-R (located at 100 bp downstream of NOS terminator) was used (Figure 1A). Amplified PCR-fragments were briefly purified with the extraction of phenol/chloroform and precipitated in ethanol [18]. In the first task, we prepared PCR- fragments of 35S-GFP-NOS from plasmid p35S-GFP. To remove residues of template plasmid DNA from the PCR-fragments, we purified PCR-fragments 35S-GFP-NOS through agarose gel extraction. Then, purified PCR-fragments 35S-GFP-NOS were transformed into epidermal cells of Onion peels with the biolistic bombardment delivery system. To compare transformation efficiency, plasmid DNA p35S-GFP was transformed in parallel. For a negative control, we transformed PCR-fragments generated from the same template plasmid but not containing GFP sequences. GFP signal was compared in transformations with PCR-fragments and with plasmid DNA. Results showed that GFP fluorescent signal was detected in the transformation with 35S- GFP-NOS or p35S-GFP, no GFP signal was detectable in the negative control (Figure S1). These results indicated that PCR- fragment based transient expression system (PCR-TES) could be used as an alternative way to assess gene expression in plant cells. Next, we compared the efficiency between PCR-fragments transformation and plasmids transformation. PCR-fragment 35S- GFP-NOS and plasmid p35S-Cherry were cotransformed into epidermal cells of Onion peels. Results showed that cells expressing Cherry fluorescent signal (red) were also displaying GFP fluores- cent signal (green) (Figure 1B), suggesting that transformation with PCR-fragments, instead of plasmid DNA, could be a substantial approach for transiently assessing a gene expression in plant cells.